This invention is directed to negative-acting photoimageable compositions which are developable in alkaline aqueous solutions. The invention is particularly applicable to primary photoimaging resists, but is applicable, as well, to compositions that are hardenable so as to form solder masks and the like.
A variety of such photoimageable compositions are described. Essential compositions of the type to which the present invention is directed are A) a binder polymer; B) photopolymerizable .alpha.,.beta.-ethylenically unsaturated compound(s), and C) a photoinitiator chemical system. The binder polymer A) has sufficient acid functionality, generally carboxylic acid functionality, that the binder polymer is soluble in alkaline aqueous solution and thereby renders the photoimageable composition developable in alkaline aqueous solutions. The photopolymerizeable compound(s) B) are monomers and/or short chain oligomers, a substantial portion of which have multiple .alpha.,.beta.-ethylenic unsaturated functionality.
The photoinitiator chemical system C) includes chemicals which generate free radicals upon exposure to actinic radiation. These free radicals propagate the polymerization of the .alpha.,.beta.-ethylenic unsaturated moieties of the photopolymerizeable compounds B). Herein, the photoinitiator system C) is deemed to include not only chemical compounds which generate free radicals, but catalysts or sensitizers which promote the free-radical initiated polymerization of the .alpha.,.beta.-ethylenic unsaturated moieties of the photopolymerizeable compounds B).
Circuit boards almost invariably have through-holes to connect circuitry on opposite faces of the board. With holes becoming larger on circuit boards, higher tenting strength is becoming increasingly important; thus greater flexibility of photoimageable compositions is required.
Improved flexibility also contributes to an improved cross hatch adhesion which allows for better compatibility with automated polyester removal systems. If the photoresist is brittle, these polyester removal systems will cause chipping of the photoresist and subsequently, circuit line defects.
By replacing a portion of conventional photo reactive monomers (like ethoxylated trimethylolpropane triacrylate) with an isocyanuric, urethane-based oligomer, a significant improvement to tenting strength and flexibility was observed. However, even though the flexibility was noticeably better, the fine line adhesion was not improved and the oligomer was shown to be a major source of developing scumming.
Improved fine line adhesion and lower developer scum has been demonstrated when the isocyanuric oligomer is comprised of the product of a polyethoxymonomethacrylate and the isocyanurate trimer of hexamethylene diisocyanate, as described, for example, in European Patent Application EP 0 738927 A2 and Japanese Patent Document JP 08-2868372. The use of the product of a polyalkoxymonomethacrylate in UV-curable photoresists enhances the performance of such compositions over those made with urethane compounds based on the isocyanurate trimer of hexamethylene diisocyanate. Present day commercial requirements include further improvements to both fine line adhesion and developer scumming.
Herein, urethane biuret oligomers are incorporated as at least a portion of the photopolymerizeable component B). The urethane biuret oligomers are found to minimize the developer scum. Along with improving the developer scum problem, it has been found that urethane biuret oligomers improve the fine line adhesion of the photoresist.